Membrane vesicles are spherical membrane microparticles, generally less than 200 nm in diameter. The microparticles are composed of a lipid bilayer containing a cytosolic fraction. Particular membrane vesicles are more specifically produced by cells, from intracellular compartments through fusion with the cytoplasmic membrane of a cell, resulting in their release into the extracellular biological fluids of an organism or into the supernatant of cells in culture. These vesicles/microparticles may be released in a number of ways. The classical secretory pathway processes mainly traditional membrane signals bearing receptors through the Endoplasmic Reticulum (ER) membrane (Lee et al., (2004) Annu. Rev. Cell Dev. Biol. 20, 87-123).
The secretory proteins are packaged into transport vesicles, delivered to the Golgi apparatus, and eventually released of into the extracellular space.
Alternatively, nonclassical secretory pathways exist and mediate translocation of cytosolic, nonsignal bearing molecules into the extracellular space (Lippincott-Schwartz et al., (1989) Cell 56, 801-813; and Misumi et al., (1986) J. Biol. Chem. 261, 11398-11403). Two of these involve intracellular vesicles of the endocytic membrane system, such as secretory lysosomes (Muesch et al., (1990) Trends Biochem. Sci. 15, 86-88) and exosomes (Johnstone et al., (1987) J. Biol. Chem. 262, 9412-9420), the latter ones being internal vesicles of late endosomes or multivesicular bodies (MVB). Lysosomal contents gain access to the exterior of cells when specialized endocytic structures such as secretory lysosomes of cytotoxic T lymphocytes fuse with the plasma membrane. Lumenal contents of late endocytic structures are released into the extracellular space when MVBs fuse with the plasma membrane resulting in release of the internal multivesicular endosomes into the extracellular space (called exosomes) along with their cargo molecules. Other nonclassical pathways involve direct translocation of cytosolic factors across the plasma membrane using protein conducting channels or a process called membrane blebbing (Nickel, W. (2005) Traffic. 6, 607-614). Membrane blebbing is characterized by shedding of plasma membrane-derived microvesicles into the extracellular space.
Microparticle release has been demonstrated from different cell types in varied physiological contexts. It has been demonstrated that tumor cells secrete microparticles, such as exosomes; texosomes, Tex or tumor exosomes (Yu et al., (2007) J. Immunol. 178, 6867-6875) in a regulated manner, carrying tumor antigens and capable of presenting these antigens or transmitting them to antigen presenting cells (patent application No. WO99/03499). These microparticles are released by tumor cells and cause immune suppression through immune cell killing or deregulation allowing tumor growth. Release of these FasL or TNF containing exosomes has been found to be one mechanism by which the tumor promotes a state of immune privilege/immune suppression. Alternatively, it has shown that HIV infected cells release Nef-containing vesicles (Guy et al., (1990) Virology 176, 413-425; and Campbell et al., (2008) Ethn. Dis. 18, S2-S9). We postulate that these vesicles are similarly used by HIV to dysregulate the immune system allowing HIV to survive. Finally, the endosomal trafficking pathway has been suggested to also be involved in virion release from infected cells (Sanfridson et al., (1997) Proc. Natl. Acad. Sci. U.S.A 94, 873-878; and Esser et al., (2001) J. Virol. 75, 6173-6182). Thus, during the HIV infection, the endosomal pathway, involved in several vesicle release pathways, serves a dual function in both regulation of the immune system and in virion release of infected cells. It would be of particular interest to have an effective method that could be used to dampen or inhibit microparticle/vesicle release.